This invention relates to an oxygen concentration detecting device which generates an output proportional to the concentration of oxygen in exhaust gases from an internal combustion engine.
An air-fuel ratio control system for an internal combustion engine is known, e.g., from Japanese Patent Publication (Kokoku) No. 55-3533, which senses oxygen concentration in exhaust gases from the engine, and controls the air-fuel ratio of a mixture supplied to the engine to a desired value in a feedback manner responsive to the detected concentration value, to thereby purify the exhaust gases and decrease the fuel consumption.
The above-mentioned oxygen concentration detecting device for use in an air-fuel ratio control system includes a type which generates an output proportional to the concentration of oxygen contained in the exhaust gases, i.e., the air-fuel ratio of the exhaust gases. An oxygen concentration detecting device of this type is disclosed, e.g., in Japanese Provisional Patent Publication (Kokai) No. 59-192955, which comprises an oxygen-pumping element and a cell element, each being composed of a plate-like member formed of a solid electrolytic material having oxygen ion-conductivity, and a couple of electrodes attached to opposite side surfaces of the plate-like member. A gas diffusion chamber is partly defined by one of the electrodes of each of the oxygen-pumping element and the cell element. A gas to be examined is introduced into the gas diffusion chamber through a gas-introducing slit. An air chamber into which the atmosphere is introduced is provided adjacent the cell element, with the other of the coupled electrodes of the cell element facing the interior of the air chamber.
According to the disclosed oxygen concentration detecting device, in order to maintain the concentration of oxygen present within the gas diffusion chamber at a predetermined value (e.g., 0), a voltage developed across the cell element is compared with a predetermined reference value, and pumping current is caused to flow between the two electrodes of the oxygen-pumping element in response to the result of the comparison. The value of the pumping current is outputted as an output proportional to the oxygen concentration in the gas to be examined.
To detect the pumping current, a current detecting resistance is used, which is connected in series to the oxygen-pumping element, voltages at opposite ends of which are utilized to obtain a voltage representing the pumping current.
According to an oxygen concentration detecting device of the proportional-output type as described above, as distinct from the so-called .alpha.=1 type oxygen concentration sensor whose output voltage abruptly changes as the air-fuel ratio changes across the stoichiometric air-fuel ratio, the pumping current I.sub.p changes linearly as the air-fuel ratio changes on the rich side or on the lean side with respect to the stoichiometric ratio, without abruptly changing in the vicinity of the stoichiometric air-fuel ratio, as shown in FIG. 1. The detection of the air-fuel ratio utilizes the linear relationship between the pumping current and the air-fuel ratio. However, in actuality, as stated before, the air-fuel ratio is determined from a voltage outputted from the detection system including the current detecting resistance, it is requisite to enhance the accuracy of conversion of the pumping current I.sub.p into the voltage in order to accurately determine the air-fuel ratio.
In the conventional device, the system for detecting the pumping current I.sub.p is formed by part of a control circuit which controls the supply of pumping current I.sub.p to the oxygen-pumping element. Any circuit error may cause noise contained in the pumping current signal, which impedes accurate detection of the air-fuel ratio and hence degrades the detection accuracy.
Further, in the case where the oxygen concentration detecting device is arranged in a place undergoing high level noise, such as external noise in ignition pulses of an internal combustion engine, it is difficult to accurately determine changes in voltages at the opposite ends of the current detecting resistance, which also forms a factor for degraded detection accuracy.
Also, in the case where the air-fuel ratio control is effected by the use of an oxygen concentration sensor of the proportional-output type in an internal combustion engine equipped with a three-way catalytic converter arranged in the exhaust system, the catalytic converter has the maximum conversion efficiency when the air-fuel ratio of a mixture supplied to the engine assumes a stoichiometric ratio (e.g., 14.7). Therefore, the air-fuel ratio is controlled so as to become equal to the stoichiometric ratio by means of feedback control responsive to the oxygen concentration sensor. Specifically, the feedback control is effected in response to an air-fuel ratio signal corresponding to the difference between the actual air-fuel ratio and the stoichiometric ratio. However, when the actual air-fuel ratio is close to the stoichiometric ratio, the difference between the two ratios is small, which necessitates amplifying the difference so as to secure a required degree of accuracy of the air-fuel ratio control. However, if the amplification degree or factor is set to too large a value, there can occur saturation of the air-fuel ratio signal, thus making it difficult to set the amplification degree.
Moreover, in the conventional oxygen concentration detecting device, the negative pole electrode of the cell element and the electrode of the oxygen-pumping element which are disposed in the gas diffusion chamber are grounded by way of respective long lead wires extending separately from each other. As a result, there is a problem of fluctuation or variation of the output of the sensor, i.e., the pumping current. This has been a bar to employment of a proportional-output type oxygen concentration sensor in an air-fuel ratio control system.
To be specific, since the grounding lead wires are disposed to separately extend over a long distance to the electrical system such as a control circuit where they are grounded, there occurs a difference in potential between the sensor body and the electrical system, i.e., a difference in potential between the two electrodes in the gas diffusion chamber. Especially, since the sensor body of the oxygen concentration detecting device is mounted in the exhaust system of the engine which is remote from the electrical system, the above potential difference cannot be prevented even if the two electrodes are grounded at the same point. Further, since feedback control is effected so as to maintain the voltage across the cell element constant, such potential difference may cause self-oscillation of the feedback system, in a high frequency range in particular. This causes fluctuations in the output from the detecting device, even if the detected air-fuel ratio remains the same. That is, it causes fluctuations in the pumping current, resulting in degraded detection accuracy.